A luminescent material, such as cesium iodide (CsI), potassium iodide (KI), rubidium iodide (RbI), gallium selenide (Ga.sub.y Se), gadolinium oxysulphate (Gd.sub.2 O.sub.2 S), lanthanum oxysulphate (La.sub.2 O.sub.2 S), cadmium sulphide (CdS), zinc cadmium sulphide (Zn.sub.x Cd.sub.1-x S), cadmium tungstate (CdWO.sub.3), or lead oxide (PbO.sub.z), will receive incident charged particles or photons of high kinetic energy and convert part or all of this kinetic energy to one or a plurality of photons of individual energies lying in the range 1-4 eV. The electromagnetic radiation emitted by the luminescent material is not wholly directed in a single forward direction, but is emitted in all directions, although not isotropically. Preferably, most or all of this radiation should propagate in approximately the forward direction, toward a photodiode layer that will provide an electrical signal indicating arrival of the incident high energy charged particles or photons. For this reason, many workers have attempted to promote forward direction emission of photons by the light-emitting atoms or molecules contained in the luminescent material.
One early approach to compartmentalization of radiation produced in a luminescent layer is disclosed by MacLeod in U.S. Pat. No. 3,041,456, where thin walls, running in two orthogonal directions, of optically transparent material are provided between thin adjacent layers or rectangular parallelepipeds of luminescent material.
Another early approach, disclosed by Ligtenberg et al in U.S. Pat. No. 3,825,763, provides a substrate of glass or metal (e.g., Al or Ti), on which a thin scintillation layer of CsI or Z.sub.x Cd.sub.1-x S is deposited of unspecified thickness. A separation layer of Al.sub.2 O.sub.3 is deposited on an exposed surface of the scintillation layer, and a photocathode layer of Cs.sub.2 Sb or similar material is deposited on an exposed surface of the separation layer. The substrate material and the scintillation material have respective thermal expansion coefficients of 2-2.5.times.10.sup.-5 /.degree.C., respectively, and the substrate is maintained at an elevated temperature T=150.degree.-200.degree. C. when the scintillation material is deposited thereon. The substrate-scintillation layer combination is then cooled to room temperature, and cracks develop in the thin scintillation layer as cooling proceeds. These cracks produce columns of scintillation material, separated by small air or vacuum gaps between adjacent columns and extending approximately perpendicularly to the substrate-scintillation layer interface. The crack structure thereby produced has a random collection of shapes and associated diameters.
Gudden et al, in U.S. Pat. No. 3,829,378, disclose use of a luminescent layer on a screen with an absorbing substance deposited thereon whose absorption decreases as position varies from the center toward the edges of the screen. This invention partly compensates for the tendency of screen brightness to decrease as one approaches an edge of the screen.
A process for making columnar structures of a luminescent layer on an X-ray screen is disclosed in U.S. Pat. No. 4,069,355 by Lubowski et al. Depressions or valleys are etched at regular intervals in an underlying substrate, and the luminescent material is grown only on the raised portions of the substrate. The gaps between adjacent columns of luminescent material are filled with a highly reflecting material or with another luminescent material.
Sonoda, in U.S. Pat. No. 4,239,791, discloses a method for making a screen image intensifier. A heated phosphorescent material layer is treated with a colder liquid material, such as acetone, to cause differential thermal contraction and form a plurality of elongated cracks in this layer running approximately perpendicular to the substrate-phosphorescent layer interface. These cracks are asserted to form optically independent columns of phosphorescent material.
Riihimaki et al disclose an X-ray intensifying screen with a luminescent layer formed, in an unspecified manner, with a plurality of regularly spaced grooves therein to capture and guide light produced in the luminescent layer. The grooves run in one direction only, and it is unclear how light is channelled within the air or vacuum gaps (with refractive index=1) between the luminescent material (with refractive index&gt;1).
Van Leunen, in U.S. Pat. No. 4,712,011, discloses use of the columnar structure produced by the Ligtenberg, et al, invention, and deposits an X-ray-absorbing material in the air/vacuum gaps to absorb X-ray light incident on a gap. Up to five percent of the weight of the scintillation layer may be X-ray-absorbing material deposited in the gaps, but no method of depositing the X-ray absorbing material is discussed.
A method for vapor deposition of a luminescent layer on a screen for image intensification is disclosed by Ligtenberg et al in U.S. Pat. No. 4,842,894. The vapor deposition crucible is positioned at about 20.degree. relative to the normal to the screen, and gaps formed between columns of the luminescent material appear to be elongated bubbles of unspecified material (possibly air or a vacuum). The luminescent material apparently forms predominantly crystalline columns of this material.
Bates, in Advances in Electronics and Electron Physics, vol. 28A (1969) pp. 451-459, discloses use of crystalline CsI and thermally-induced cracking of a contiguous substrate, with crack diameter about 0.5 .mu.m. It is unclear whether the subsequently grown CsI forms into spaced apart columns as a result of presence of the substrate cracks.
Stevels and Schrama-dePauw discuss some characteristics of vapor deposited CsI, activated with Na, in Philips Research Reports, vol. 29 (1974) pp. 341-352 and 353-362. Cracks are thermally induced in a substrate, held at a temperature of T=50.degree.-300.degree. C., and a thick or thin CsI(Na) layer is subsequently grown on a cracked surface of the substrate. Stevels et al discuss the effects of heat treatment, thick versus thin CsI layers, the substrate material (KI, RbI or other) and average diameter of the cracks on columns of CsI that form on the substrate surface. The possibility of light channeling in such columns is discussed.
An X-ray image intensifier, using CsI material formed into irregular columns by a cracked mosaic pattern on a substrate, is disclosed by Washida and Sonoda in Advances in Electronics and Electron Physics, vol. 52 (1979) pp. 201-207. The possibility of X-ray channeling is discussed, and two types of column spacings are discussed. Maximum improvement of X-ray intensity, relative to use of a conventional, non-columnar luminescent material, appears to be about 40 percent.
British Patent No. 1,423,935, issued to Philips Electronics and Associated Industries, Ltd., discloses provision of a mosaic crack structure in the form of circles, hexagons and rectangles on a substrate surface. Cesium iodide or a similar luminescent material is subsequently vapor deposited on this surface. It is unclear whether spaced apart columns of this vapor deposited material form as a result of the mosiac crack structure.
Most or all of the work discussed above relies upon an irregular mosaic crack structure, formed by thermal mismatch or a similar process, to provide formation of columns, if any, of a deposited luminescent material. Little or no regularity or control is available for parameters such as column diameters, spacing of adjacent columns, or the tendency of such columns to coalesce if the luminescent material that provides control of these parameters and allows flexibility in formation of such structures.